Bulkhead fittings are a particular type of fitting used to create a sealed, mechanical connection through a wall, typically of a tank, or other fluid-containing vessel. The threaded versions of these fittings may include a body, a gasket, and a nut. Flange-type bulkhead fittings are also commercially available. Bulkhead fittings are typically constructed from metals and thermoplastics. The prior art includes U.S. Pat. No. 5,551,590, for example, which discloses a non-metallic pressure vessel fitting.
Tanks, especially those manufactured from thermoplastics, react mechanically to thermal changes due to temperature changes in the surrounding atmosphere, changes in temperature of the fluid contained in the tank, and changes due to solar heating. Additionally, tanks react mechanically to changes in internal pressure and changes in static head pressure of the fluid contained in the tank. These mechanical reactions can be generally described as (a) an expansion, or increase, in the tank dimensions due to increases in temperature and/or pressure, and (b) contraction, or a decrease, in tank dimensions due to decreases in temperature and/or pressure. Expansion of the tank dimensions results in a reduction of the wall thickness of the vessel, while contraction of the tank dimensions results in an increase of the wall thickness of the vessel.
As thermoplastic tank dimensions change, especially wall thickness, the initial compression imparted on the gasket may be affected. In particular as the tank expands, reducing wall thickness, a subsequent reduction in the initial compression imparted on the gasket may result in loosening of the fitting with the potential for a leak forming around the gasket. Further, as gasket compression is reduced by the continuous cycles of expansion and contraction, the load initially imparted between the threads of the nut and the threads of the body may be reduced. This may also result in the loosening of the fitting and the formation of potential leaks around the gasket.
Further, if the bulkhead fitting is manufactured from thermoplastic materials, the bulkhead fitting is subject to the same expansion and contraction changes that are imparted on the tank. Thus, the bulkhead fitting will be expanding at the same time the wall of the tank is getting thinner, exacerbating the decrease in the initial compression imparted on the gasket. If the fitting is manufactured from a thermoplastic material that has a coefficient of thermal expansion and contraction substantially different from the tank material, dimensional changes due to thermal or pressure changes may result in a further decrease in the initial compression imparted on the gasket. Additionally, the thermal expansion of the piping system attached to the bulkhead fitting can be of concern.
Also, if a bulkhead fitting is exposed to constant and excessive vibration, independent of, or combined with, physical changes in the tank due to thermal, or pressure changes, the bulkhead fitting may become loose, especially the nut portion of the bulkhead fitting. What is needed in the art is a bulkhead fitting that maintains a substantially constant pressure or constant compression on the gasket, in multiple conditions of use.